Injectors and methods of this type are known and usually include preselection of an actuator voltage to which the piezoelectric actuator is to be charged and/or recharged to move the valve needle of the injector to a desired position and/or to put the injector in a desired operating state. However, because of aging effects in the piezoelectric actuator itself in particular as well as the mechanical and hydraulic components in the injector, there are changes in the corresponding electrical and/or mechanical parameters of the injector, so that accurate metering of a quantity of fuel to be injected, for example, is impossible over the long term using the known methods. In addition to these aging effects, temperature fluctuations in the area of the injector in particular also cause a change in the electric capacitance of the piezoelectric actuator, resulting in additional inaccuracies in metering fuel or other fluids through the injector or in positioning the actuator in general. In addition, part variations among various injectors, which are assigned to different cylinders of a certain internal combustion engine, for example, result in individual deviations in fuel injection for each cylinder, which are also unwanted.